1. Field of the Invention
The present invention is related to a backup power supply system with a null transfer time. More particularly, the present invention is applied to a dc/ac power inverter for avoiding a short period of power interruption during the duration of the power transfer caused by the mechanical switch operation in the output terminal of the dc/ac power inverter.
2. Description of the Related Art
Referring to FIG. 1, a conventional backup power supply system (I) 10 is connected to a first ac power source 2 and a dc power source 3 at its upstream side. The conventional backup power supply system (I) 10 is further connected to a load 4 at its downstream side. The backup power supply system (I) 10 includes a first switch (S11) 101, a second switch (S12) 102 and a dc/ac power inverter 104. The dc/ac power inverter 104 converts the dc power supplied from the dc power source 3 into an ac power to build up a second ac power source 105. When the first ac power source 2 is operated normally, the first switch (S11) 101 is closed and the second switch (S12) 102 is opened. Thus, the first ac power source 2 directly supplies electric power to the load 4 and the dc/ac power inverter 104 is operated in the condition of the hot standby. When the first ac power source 2 is failed, the first switch (S11) 101 is opened and the second switch (S12) 102 is closed. In this time, the electric power demanded by the load 4 is supplied from the dc power source 3 through the backup power supply system (I) 10. Generally, the first switch (S11) 101 and the second switch (S12) 102 are static switches, such as TRIAC or a pair of SCRs (Silicon Controlled Rectifiers) parallel connected in a reverse connection. The static switch only requires a few microseconds to process the switching operation that may result in the power interruption. However, a few microseconds power interruption will not affect the normal operation of sensitive electric facilities. Hence, the backup power supply system (I) 10 can completely avoid the power interruption when power supply for the load 4 is switched from the first ac power source 2 to the second ac power source 105. However, it will result in a voltage drop on the static switch (generally 1-2 V contrast to null voltage drop of a mechanical switch) while turning on the static switch. In comparison with the mechanical switch, the static switch applied to the output side of dc/ac power inverter 104 will result in larger power loss and lower efficiency. To cool down the static switches, a heat-dissipation apparatus is required. Consequently, the backup power supply system (I) 10 has the disadvantage mentioned above although the static switch of the second switch (S12) 102 can accomplish the backup power supply system (I) 10 with a null transfer time.
Referring to FIG. 2, another conventional backup power supply system (II) 20 is connected to a first ac power source 2 and a dc power source 3 at its upstream side. The conventional backup power supply system (II) 20 is further connected to a load 4 at its downstream side. The backup power supply system (II) 20 includes a first switch (S21) 201, a second switch (S22) 202, a third switch (S23) 203 and a dc/ac power inverter 204. The dc/ac power inverter 204 converts the dc power supplied from the dc power source 3 into an ac power to build up a second ac power source 205. Wherein the first switch (S21) 201 and the second switch (S22) 202 are static switches; the third switch (S23) 203 is a mechanical switch, such as a relay or an electromagnetic contactor. When the first ac power source 2 is operated normally, the first switch (S21) 201 is closed, and the second switch (S22) 202 as well as the third switch (S23) 203 are opened. Thus, the first ac power source 2 directly supplies electric power to the load 4 and the dc/ac power inverter 204 is operated in the condition of the hot standby. When the first ac power source 2 is failed, the first switch (S21) 201 is opened, and the second switch (S22) 202 as well as the third switch (S23) 203 are closed. In this time, the electric power demanded by the load 4 is supplied from the dc power source 3 through the backup power supply system (II) 20. The first switch (S21) 201 and the second switch (S22) 202 are static switches. The static switch only requires a few microseconds to process the switching operation that may cause power interruption. However, a few microseconds power interruption will not affect the normal operation of sensitive electric facilities. Hence, the backup power supply system (II) 20 can completely avoid the power interruption when power supply for the load 4 is switched from the first ac power source 2 to the second ac power source 205. After a few microseconds, the third switch (S23) 203 with a relatively slow speed for switching has already been closed since the turn-on time of the mechanical contactor of the third switch (S23) 203 is longer than that of the second switch (S22) 202. Thus, the third switch (23) 203 acts as a bypass of the second switch (22) 202 that may avoid the voltage drop mentioned above and the power loss caused by the static switch employed in the backup power supply system 10. In comparison with the backup power supply system 10, the backup power supply system (II) 20 may increase the manufacturing cost due to the additional elements and increased dimensions. Besides, the reliability of the backup power supply system (II) 20 may be reduced due to the additional elements.
The present invention intends to provide a backup power supply system with a null transfer time by controlling the dc/ac power inverter. Whether the first power source is in a normal or abnormal condition, the output switch of the dc/ac power inverter is continuously closed. Once the first power source is abnormal or has failed, the dc/ac power inverter rapidly converts the power of the dc power source and supplies a second ac power source to a load avoiding a short period of power interruption during the power transfer duration. In this circumstance, the backup power supply system employs a mechanical switch rather than a static switch in the output side of dc/ac power inverter, and thus can improve the voltage drop of switches, increase the entire power efficiency, and prevent overheating problems etc.